1. Field of the Invention
The invention generally relates to surface acoustic wave devices and methods of fabricating the same, and more particularly, to a surface acoustic wave device having a SAW chip hermetically sealed, and a method of fabricating the same.
2. Description of the Related Art
Recently, there has been a demand to downsize electric parts mounted to electronic devices and improve the performance thereof with downsizing and high performance of the electronic devices. For instance, there have been similar demands on surface acoustic wave (SAW) devices that are electric parts used as filters, delay lines, oscillators in electronic devices capable of transmitting and receiving radio waves.
A description will now be given of a filter device equipped with a conventional SAW device. FIG. 1A is a perspective view of a SAW filter 100, and FIG. 1B is a cross-sectional view taken along a line F-F shown in FIG. 1A. This type of SAW device is disclosed, for example, Japanese Patent Application Publication No. 8-18390 (see FIG. 4, particularly).
Referring to FIG. 1A, the SAW filter 100 includes a ceramic package 102 having a cavity 109, a metal cap 103 and a SAW chip 110. The SAW chip 110 is placed in the cavity 109, which is sealed with the metal cap 103. As shown in FIG. 1B, the package 102 has a three-layer structure composed of three joined substrates 102a, 102b and 102c. Electrode pads 105 are provided on the top of the substrate 102b, and foot patterns 107 are provided on the bottom of the substrate 102c. Wiring patterns 106 are provided on sides of the package 102, and connect the electrode pads 105 and the foot patterns 107. The SAW chip 110 is fixed to the bottom of the cavity 109 so that comb-like electrodes (an interdigital transducer: IDT) 113 on the SAW chip 110 face up. Electrode pads 114 on the SAW chip 110 are connected to the electrode pads 105 via wires 108. The metal cap 103 is bonded to the top surface of the package by a bonding material made of solder or resin, which material serves as a washer 104.
There is another proposal to mount the SAW chip in flip-chip fashion (see, for example, Japanese Patent Application Publication No. 2001-110946). FIGS. 2A and 2B show this type of SAW device. More particularly, FIG. 2A is a perspective view of a SAW chip 210 of a SAW filter 200, and FIG. 2B is a cross-sectional view of the SAW filter 200, which view corresponds to a cross section taken along the line F-F shown in FIG. 1A.
As shown in FIG. 2A, the SAW chip 210 has a piezoelectric material substrate (hereinafter referred to as piezoelectric substrate) 211. Comb-like electrodes 213 that form an IDT are formed on a main surface (upper surface) of the piezoelectric substrate 211. Electrode pads 214 are provided on the main surface and are electrically connected to the IDT 213 via wiring patterns. As shown in FIG. 2B, a package 202 has a cavity 209. Electrode pads 205 are provided on the bottom of the cavity 209, which is also referred to as die-attached surface. The pads 205 are positioned so as to correspond to the electrode pads 214 of the SAW chip 211. The SAW chip 210 is flip-chip mounted in the cavity 209 so that the IDT 213 and the electrode pads 214 face the die-attached surface. The electrode pads 214 and 205 are bonded via metal bumps 208 so that these pads are electrically and mechanically fixed together. The electrode pads 205 are electrically connected to foot patterns 207 on the backside of the package 202 by means of via-wiring lines 206, which penetrate the bottom portion of the package 202. A metal cap 203 closes an opening of the cavity 209 and is bonded to the package 202 by a bonding material 204.
A duplexer equipped with a transmit filter and a receive filter may be formed by using SAW filters as mentioned above. Such a duplexer will now be described with reference to FIGS. 3A and 3B. A duplexer 300 shown in these figures has a transmit filter 310a and a receive filter 310b, each of which filters is like the SAW filter 100. FIG. 3A shows a cross section of the duplexer 300, which corresponds to that taken along the line F-F shown in FIG. 1A. FIG. 3B is a plan view of a SAW chip 310.
Referring to FIG. 3A, the duplexer 300 has a package 302 in which the SAW chip 310 is mounted. A matching-circuit board 321 and a main board 322 are provided on the bottom side of the package 302. The matching-circuit board 321 is provided in such a way as to be sandwiched by the main board 322. As shown in FIG. 3B, the SAW chip 310 is equipped with the transmit filter 310a and the receive filter 310b. Each of the filters 310a and 310b has respective IDTs 313 arranged in ladder fashion. The IDTs 313 are connected to electrode pads 314 via wiring patterns 315.
The SAW filter or duplexer as mentioned above is required to have the SAW chip hermetically sealed. The metal cap is used, along with bonding material or resin, to accomplish hermetically sealing.
However, there are drawbacks to be solved. A large joining area (seal width) at the interface between the package and the cap is needed to hermetically seal the cavity with high reliability. However, this prevents downsizing of the package. Downsizing of package is also restricted due to the use of wires because the wires need a relatively wide pattern for bonding. The package is the multilayer substrate made of ceramics, which is comparatively expensive. The device needs the process of assembling the cap, chip and package device, and is therefore costly.
It is an object of the present invention to provide a downsized, less expensive, productive SAW device and a method of fabricating the same.
This object of the present invention is achieved by a surface acoustic wave device comprising: a piezoelectric substrate having a first surface on which comb-like electrodes, first pads connected thereto, and a first film are provided, the first film being located so as to surround the comb-like electrodes; and a base substrate having a second surface on which second pads joined to the first pads and a second film joined to the first film are provided, the first and second films joined by a surface activation process defining a cavity in which the comb-like electrodes and the first and second pads are hermetically sealed.
The above objects of the present invention are also achieved by a method of fabricating a surface acoustic wave device comprising the steps of: (a) forming a first film on a first surface of a piezoelectric substrate on which comb-like electrodes and first pads are formed so as to be surround by the first film; (b) forming a second film on a second surface of a base substrate, the second film corresponding to the first film in position; (c) subjecting a surface activation process to surfaces of the first and second films; and (d) joining the first and second films so as to join activated surfaces thereof, the comb-like electrodes being hermetically sealed in a cavity defined by the first and second films.